(Dundee, 1867); "Tertiary Volcanic Rocks of the British Isles," _Quart.
Journ. Geol. Soc._, vol. xxvii. p. 279; also, "History of Volcanic
Action in British Isles," _Trans. Roy. Soc. Edin._ (1888); Judd, "On the
Ancient Volcanoes of the Highlands," etc., _Quart. Journ. Geol. Soc._,
vol. xxx. p. 233; and _Volcanoes_, p. 139.
[2] _Brit. Assoc. Rep._ for 1850, p. 70.
[3] Judd, _Quart. Jour. Geol. Soc._, vol. xxx. p. 242.
[4] _History of Volcanic Action, etc._, _loc. cit._ p. 153, _et seq._
The "Granophyres" of Geikie come under the head of "Felsites," passing
into "granite" in one direction and quartz-trachyte in another,
according to Judd; the proportion of silica from 69 to 75 per
cent.--_Quart. Jour. Geol. Soc._, vol. xxx. p. 235.
[5] This view the author has expressed in a recent edition of _The
Physical Geology of Ireland_, p. 177 (1891).
CHAPTER IV.
ISLE OF SKYE.
This is the largest and most important of all the Tertiary volcanic
districts, but owing to the extensive denudation to which, in common
with other Tertiary volcanic regions of the British Isles, it has been
subjected, its present limits are very restricted comparatively to its
original extent. Not only is this evident from the manner in which the
basaltic sheets terminate along the sea-coast in grand mural cliffs, as
opposite "Macleod's Maidens," and at the entrance to Lough Bracadale on
the western coast, but the evidence is, according to Sir A. Geikie,
still more striking along the eastern coast; showing that the Jurassic,
and other older rocks there visible, were originally buried deep under
the basaltic sheets which have been stripped from off that part of the
country. These great plateau-basalts occupy about three-fourths of the
entire island along the western and northern areas, rising into terraced
mountains over 2,000 feet in height, and are deeply furrowed by glens
and arms of the sea, along which the general structure of the tableland
is laid open, sometimes for leagues at a time.
It is towards the south-eastern part of the island that the most
interesting and important phenomena are centred; for here we meet with
representatives of the acid (or highly silicated) group of rocks, and
of remarkable beds of gabbro, which have long attracted the attention of
petrologists. These latter beds, throughout a considerable distance
round the flanks of the Cuillin Hills, are interposed between the acid
rocks and the plateau-basalts; but towards the north, on approaching
Lough Sligahan, the acid rocks, consisting of granophyres,
quartz-porphyries, and hornblendic-granitites, are in direct contact
with the plateau-basalts; and, according to the very circumstantial
account of Sir A. Geikie, are intrusive into them; not only sending
veins into the basaltic sheets, but also producing a marked alteration
in their structure where they approach the newer intrusive mass. Equally
circumstantial is the same author's account of the relations of the
granophyres to the gabbros,[1] as seen at Meall Dearg and the western
border of the Cuillin Hills--where the former rock may be seen to send
numerous veins into the latter. Not only is this so, but the granophyre
is frequently seen to truncate, and abruptly terminate some of the
basaltic dykes by which the basic sheets are traversed--as in the
neighbourhood of Beinn na Dubhaic. All these phenomena strongly remind
us of the conditions of similar rocks amongst the mountains of Mourne
and Carlingford in Ireland; where, at Barnaveve, the syenite (or
hornblendic quartz-felsite) is seen to break through the masses of
olivine gabbro, and send numerous veins into this latter rock.[2]
The interpretation here briefly sketched differs widely from that
arrived at by Professor Judd. The granitoid masses of the Red Mountains
(Beinn Dearg) and the neighbouring heights are, in his view, the roots
of the great volcano from which were erupted the various lavas; the
earlier eruptions producing the acid lavas, to be followed by the
gabbros, and these by the plateau-basaltic sheets, which stretch away
towards the north and west into several peninsulas. Thus he holds that
"the rocks of basic composition were ejected subsequently to those of
the acid variety," and appeals to various sections in confirmation of
this view.[3] To reconcile these views is at present impossible; but as
the controversy between these two observers is probably not yet closed,
there is room for hope that the true interpretation of the relations of
these rocks to each other will ere long be fully established.
[1] Geikie, _loc. cit._, p. 161, etc.
[2] _Physical Geology of Ireland_, 2nd edition, p. 174 (Fig. 21).
Professor Judd has also come to the conclusion that the granite of
Mourne is of Tertiary age, _Quart. Jour. Geol. Soc._, vol. xxx. p. 275.
[3] Judd, _loc. cit._, p. 254.
CHAPTER V.
THE SCUIR OF EIGG.
Amongst the more remarkable of the smaller islets are those of Eigg,
Rum, Canna, and Muck, lying between Mull on the south and Skye on the
north, and undoubtedly at one time physically connected together. The
Island of Eigg is especially remarkable for the fact, as stated by
Geikie, that here we have the one solitary case of "a true superficial
stream of acid lava--that of the Scuir of Eigg."[1] (Fig. 34.) This
forms a sinuous ridge, composed of pitchstone of several kinds, of over
two miles in length, rising from the midst of a tableland of bedded
basalt and tuff to a height of 1,289 feet above the ocean; the
plateau-basalt is traversed by basaltic dykes, ranging in a N.W.-S.E.
direction. But what is specially remarkable is the evidence afforded by
an examination of the course of the Scuir, that it follows the channel
of an ancient river-valley, which has been hollowed out in the surface
of the plateau. The course of this channel is indicated by the presence
of a deposit of river-gravel, which in some places forms a sort of
cushion between the base of the Scuir and the side of the channel. Over
this gravel-bed the viscous pitchstone-lava appears to have flowed,
taking possession of the river-channel, and also of the beds of several
small tributary streams which flowed into the channel of the Scuir. The
recent date of the pitchstone forming this remarkable mural ridge, once
occupying the bed of a river-channel, is shown by the fact that the
basaltic dykes which traverse the plateau-basalts are truncated by the
river-gravel, which is, therefore, more recent; and, as we have seen,
the pitchstone stream is more recent than the river-gravel. But at the
time when this last volcanic eruption took place, the physical geography
of the whole region must have been very different from that of the
present time. From the character and composition of the pebbles in the
old river-bed, amongst which are Cambrian sandstone, quartzite,
clay-slate, and white Jurassic limestone, Sir A. Geikie concludes that
when the river was flowing, the island must have been connected with the
mainland to the east where the parent masses of these pebbles are found.
[Illustration: Fig. 34.--View of the Scuir of Eigg from the east. The
lower portion of the mountain is formed of bedded basalt, or dolerite
with numerous dykes and veins of basalt, felstone, and pitchstone; the
upper cliff, or Scuir, is composed of pitchstone of newer age, the
remnant of a lava flow which once filled a river channel in the basaltic
sheets. A dyke, or sheet, of porphyry is seen to be interposed between
the Scuir and the basaltic sheets.--(After Geikie.)]
_Effects of Denudation._--The position of the Scuir of Eigg and its
relations to the basaltic sheets show the enormous amount of denudation
which these latter have undergone since the stream of pitchstone-lava
filled the old river channel. The walls, or banks, of the channel have
been denuded away, thus converting the pitchstone casting into a
projecting wall of rock. That it originally extended outwards into the
ocean to a far greater distance than at present is evident from the
abruptly truncated face of the cliff; and yet this remarkable volcanic
mass seems to have been, perhaps, the most recent exhibition of volcanic
action to be found in the British Isles. It is perhaps, on this account,
the most striking of the numerous examples exhibited throughout the West
of Scotland and the North-east of Ireland of the enormous amount of
denudation to which these districts have been subjected since the
extinction of the volcanic fires; and this at a period to which we
cannot assign a date more ancient than that of the Pliocene. Yet, let us
consider for a moment to what physical vicissitudes these districts have
been subjected since that epoch. Assuming, as we may with confidence,
that the volcanic eruptions were subaerial, and that the tracts covered
by the plateau-basalts were in the condition of dry land when the
eruptions commenced, in this condition they continued in the main
throughout the period of volcanic activity. But the eruptions had
scarcely ceased, and the lava floods and dykes become consolidated,
before the succeeding glacial epoch set in; when the snows and glaciers
of the Scottish Highlands gradually descending from their original
mountain heights, and spreading outwards in all directions, ultimately
enveloped the whole of the region we are now considering until it was
entirely concealed beneath a mantle of ice moving slowly, but
irresistibly, outwards towards the Atlantic, crossing the deep channels,
such as the Sound of Mull and the Minch, climbing up the sides of
opposing rocks and islands until even the Outer Hebrides and the
North-east of Ireland were covered by one vast mantle of ice and snow.
The movement of such a body of ice over the land must have been attended
with a large amount of abrasion of the rocky floor; nor have the
evidences of that abrasion entirely disappeared even at the present day.
We still detect the grooves and scorings on the rock-surfaces where they
have been protected by a coating of boulder clay; and we still find the
surface strewn with the blocks and _debris_ of that mighty ice-flood.
But whatever may have been the amount of erosion caused by the great
ice-sheet, it was chiefly confined to the more or less horizontal
surface-planes. Erosion of another kind was to succeed, and to produce
more lasting effects on the configuration of the surface. On the
disappearance of the ice-sheet, an epoch characterised by milder
conditions of climate set in. This was accompanied by subsidence and
submersion of large tracts of the land during the Interglacial stage; so
that the sea rose to heights of several hundred feet above the present
level, and has left behind stratified gravels with shells at these
elevations in protected places. During this period of depression and of
subsequent re-emergence the wave-action of the Atlantic waters must have
told severely on the coast and islands, wearing them into cliffs and
escarpments, furrowing out channels and levelling obstructions. Such
action has gone on down to the present day. The North-west of Scotland
and of Ireland has been subjected throughout a very lengthened period to
the wear and tear of the Atlantic billows. In the case of the former,
the remarkable breakwater which nature has thrown athwart the North-west
Highlands in the direction of the waves, forming the chain of islands
constituting the Outer Hebrides, and composed of very tough Archaean
gneiss and schist, has done much to retard the inroads which the waves
might otherwise have made on the Isle of Skye; while Coll and Tiree,
composed of similar materials, have acted with similar beneficent effect
for Mull and the adjoining coasts. But such is the tremendous power of
the Atlantic billows when impelled by westerly winds, that to their
agency must be mainly attributed the small size of the volcanic
land-surfaces as compared with their original extent, and the formation
of those grand headlands which are presented by the igneous masses of
Skye, Ardnamurchan, and Mull towards the west. Rain and river action,
supplemented by that of glaciers, have also had a share in eroding
channels and wearing down the upper surface of the ground, with the
result we at present behold in the wild and broken scenery of the Inner
Hebrides and adjoining coast.
[1] Geikie, _loc. cit._, p. 178; also _Quart. Jour. Geol. Soc._, vol.
xxvii. p. 303.
CHAPTER VI.
ISLE OF STAFFA.
Reference has been made to this remarkable island in a former page, but
some more extended notice is desirable before leaving the region of the
Inner Hebrides. Along with the islands of Pladda, Treshnish, and
Blackmore, Staffa is one of the outlying volcanic islands of the group,
being distant about six miles from the coast of Mull, and indicates the
minimum distance to which the plateau-basaltic sheets originally
extended in the direction of the old marginal lands of Tiree and Coll.
The island consists of successive sheets of bedded basaltic lava, with
partings of tuff, one of which of considerable thickness is shown to lie
at the base of the cliff on the south-west side of the island.[1] The
successive lava-sheets present great varieties of structure, like those
on the north coast of Antrim; some being amorphous, others columnar,
with either straight or bent columns. The lava-sheet out of which
Fingal's Cave is excavated consists of vertical prisms, beautifully
formed, and surmounted by an amorphous mass of the same material. At the
entrance of the Boat Cave we have a somewhat similar arrangement of the
columns;[2] but at the Clam-shell Cave the prisms are curved, indicating
some movement in the viscous mass before they had been fully
consolidated.
Fingal's Cave is called after the celebrated prince of Morvern (or
Morven), a province of ancient Caledonia. He is supposed to have been
the father of Ossian, the Celtic bard rendered famous by Macpherson. The
cave, one of many which pierce the coast-cliffs of Western Scotland, is
227 feet in length, 166 feet in height, and 40 feet in width. On all
sides regular columns of basalt, some entire, others broken, rise out of
the water and support the roof. The cave is only accessible in calm
weather.
[1] A drawing of this cliff is given by Geikie in the _Manual of
Geology_ (Jukes and Geikie), 3rd edition, p. 277.
[2] Prestwich, _Geology_, vol. i. p. 281, where a view of this cave is
given.
PART V.
PRE-TERTIARY VOLCANIC ROCKS.
CHAPTER I.
THE DECCAN TRAP-SERIES OF INDIA.
The great outpourings of augitic lava of Tertiary and recent times which
we have been considering appear to have been anticipated in several
parts of the world, more especially in Peninsular India and in Africa,
and it is desirable that we should devote a few pages to the description
of these remarkable volcanic formations, as they resemble, both in their
mode of occurrence and general structure, some of the great lava-floods
of a more recent period we have been considering. Of the districts to be
described, the first which claims our notice is the Deccan.
(_a._) _Extent of the Volcanic Plateau._--The volcanic plateau of the
Deccan stretches from the borders of the Western Ghats and the sea-coast
near Bombay inland to Amarantak, at the head of the Narbudda River
(long. 82 deg. E.), and from Belgaum (lat. 15 deg. 31' N.) to near Goona (lat.
24 deg. 30'). The vast area thus circumscribed is far from representing the
original extent of the tract overspread by the lava-floods, as outlying
fragments of these lavas are found as far east as long. 84 deg. E. in one
direction, and at Kattiwar and Cutch in another. The present area,
however, is estimated to be not less than 200,000 square miles.[1]
(_b._) _Nature and Thickness of the Lava-flows._--This tract is
overspread almost continuously by sheets of basaltic lava, with
occasional bands of fresh-water strata containing numerous shells,
figured and described by Hislop, and believed by him to be of Lower
Eocene age. The lava-sheets vary considerably in character, ranging from
finest compact basalt to coarsely crystalline dolerite, in which olivine
is abundant. The columnar structure is not prevalent, the rock being
either amorphous, or weathering into concentric shells. Volcanic ash, or
bole, is frequently found separating the different lava-flows; and in
the upper amygdaloidal sheets numerous secondary minerals are found,
such as quartz, agate and jasper, stilbite and chlorite. The total
thickness of the whole series, where complete, is about 6,000 feet,
divided as follows:
1. Upper trap; with ash and inter-trappean beds 1,500 feet
2. Middle trap; sheets of basalt and ash 4,000 "
3. Lower trap; basalt with inter-trappean beds 500 "
--------
6,000 "
========
Throughout the region here described these great sheets of volcanic rock
are everywhere approximately horizontal, and constitute a table-land of
3,000 to 4,000 feet in elevation, breaking off in terraced escarpments,
and penetrated by deep river-valleys, of which the Narbudda is the most
important. The foundation rock is sometimes metamorphic schist, or
gneiss, at other times sandstone referred by Hislop to Jurassic age; and
in no single instance has a volcanic crater or focus of eruption been
observed. But outside the central trappean area volcanic foci are
numerous, as in Cutch, the Rajhipla Hills and the Lower Narbudda valley.
The original excessive fluidity of the Deccan trap is proved by the
remarkable horizontality of the beds over large areas, and the extensive
regions covered by very thin sheets of basalt or dolerite.
(_c._) _Geological Age._--As regards the geological age of this great
volcanic series much uncertainty exists, owing to the absence of marine
forms in the inter-trappean beds. One single species, _Cardita
variabilis_, has been observed as occurring in these beds, and in the
limestone below the base of the trap at Dudukur. The _facies_ of the
forms in this limestone is Tertiary; but there is a remarkable absence
of characteristic genera. On the other hand, Mr. Blanford states that
the bedded traps are seen to underlie the Eocene Tertiary strata with
_Nummulites_ in Guzerat and Cutch,[2] which would appear to determine
the limit of their age in one direction. On balancing the evidence,
however, it is tolerably clear that the volcanic eruptions commenced
towards the close of the Cretaceous period, and continued into the
commencement of the Tertiary, thus bridging over the interval between
the two epochs; and since the greater sheets have been exposed
throughout the whole of the Tertiary and Quarternary periods, it is not
surprising if they have suffered enormously from denuding agencies, and
that any craters or cones of eruption that may once have existed have
disappeared.
[1] The Deccan Traps have been described by Sykes, _Geol. Trans._, 2nd
Series, vol. iv.; also Rev. S. Hislop, "On the Geology of the
Neighbourhood of Nagpur, Central India," _Quart. Journ. Geol. Soc._,
vol. x. p. 274; and _Ibid._, vol. xvi. p. 154. Also, H. B. Medlicott and
W. T. Blanford, _Manual of the Geology of India_, vol. i. (1879).
[2] Blanford, _Geology of Abyssinia_, p. 185.
CHAPTER II.
ABYSSINIAN TABLE-LANDS.
Another region in which the volcanic phenomena bear a remarkable analogy
to those of Central India, just described, is that of Abyssinia. Nor are
these tracts so widely separated that they may not be considered as
portions of one great volcanic area extending from Abyssinia, through
Southern Arabia, into Cutch and the Deccan, in the one direction, while
the great volcanic cones of Kenia and Kilimanjaro, with their
surrounding tracts of volcanic matter, may be the extreme prolongations
in the other. Along this tract volcanic operations are still active in
the Gulf of Aden; and cones quite unchanged in form, and evidently of
very recent date, abound in many places along the coast both of Arabia
and Africa. The volcanic formations of this tract are, however, much
more recent than those which occupy the high plateaux of Central and
Southern Abyssinia of which we are about to speak.
(_a._) _Physical Features._--Abyssinia forms a compact region of lofty
plateaux intersected by deep valleys, interposed between the basin of
the Nile on the west, and the low-lying tract bordering the Red Sea and
the Indian Ocean on the east. The plateaux are deeply intersected by
valleys and ravines, giving birth to streams which feed the head waters
of the Blue Nile (Bahr el Arak) and the Atbara. Several fine lakes lie
in the lap of the mountains, of which the Zana, or Dembia, is the
largest, and next Ashangi, visited by the British army on its march to
Magdala in 1868, and which, from its form and the volcanic nature of the
surrounding hills, appears to occupy the hollow of an extinct crater.
The table-land of Abyssinia reaches its highest elevation along the
eastern and southern margin, where its average height may be 8,000 to
10,000 feet; but some peaks rise to a height of 12,000 to 15,000 feet in
Shoa and Ankobar.[1]
(_b._) _Basaltic Lava Sheets._--An enormous area of this country seems
to be composed of volcanic rocks chiefly in the form of sheets of
basaltic lava, which rise into high plateaux, and break off in
steep--sometimes precipitous--mural escarpments along the sides of the
valleys. These are divisible into the following series:--
(1) _The Ashangi Volcanic Series._--The earliest forerunners of the more
recent lavas seem to have been erupted in Jurassic times, in the form of
sheets of contemporaneous basalt or dolerite amongst the Antola
limestones which are of this period. But the great mass of the volcanic
rocks are much more recent, and may be confidently referred to the late
Cretaceous or early Tertiary epochs. Their resemblance to the great
trappean series of Western India, even in minute particulars, is
referred to by Mr. Blanford, who suggests the view that they belong to
one and the same great series of lava-flows extruded over the surface of
this part of the globe. This view is inherently probable. They consist
of basalts and dolerites, generally amygdaloidal, with nodules of agate
and zeolite, and are frequently coated with green-earth (chlorite). Beds
of volcanic ash or breccia also frequently occur, and often contain
augite crystals. At Senafe, hills of trachyte passing into claystone and
basalt were observed by Mr. Blanford, but it is not clear what are their
relations to the plateau-basaltic sheets.[2]
(2) _Magdala Volcanic Series._--This is a more recent group of volcanic
lavas, chiefly distinguished from the lower, or Ashangi, group, by the
occurrence of thick beds of trachyte, usually more or less crystalline,
and containing beautiful crystals of sanidine. The beds of trachyte
break off in precipitous scarps, and being of great thickness and
perfectly horizontal, are unusually conspicuous. Mr. Blanford says, with
regard to this group, that there is a remarkable resemblance in its
physical aspect to the scenery of the Deccan and the higher valleys of
the Western Ghats of India, but the peculiarities of the landscape are
exaggerated in Abyssinia. Many of the trachytic beds are brecciated and
highly columnar; sedimentary beds are also interstratified with those of
volcanic origin. The Magdala group is unconformable to that of Ashangi
in some places. A still more recent group of volcanic rocks appears to
occur in the neighbourhood of Senafe, consisting of amorphous masses of
trachyte, often so fine-grained and compact as to pass into claystone
and to resemble sandstone. At Akub Teriki the rocks appear to be in the
immediate vicinity of an ancient vent of eruption.
From what has been said, it will be apparent that Abyssinia offers
volcanic phenomena of great interest for the observer. There is
considerable variety in the rock masses, in their mode of distribution,
and in the scenery which they produce. The extensive horizontal sheets
of lava are suggestive of fissure-eruption rather than of eruption
through volcanic craters; and although these may have once been in
existence, denudation has left no vestiges of them at the present day.
In all these respects the resemblance of the volcanic phenomena to those
of Peninsular India is remarkably striking; it suggests the view that
they are contemporaneous as regards the time of their eruption, and
similar as regards their mode of formation.
[1] W. T. Blanford, _Geology of Abyssinia_, pp. 151-2.
[2] Blanford, _loc. cit._, p. 182.
CHAPTER III.
CAPE COLONY.
_Basalt of the Plateau._--The extensive sheets of plateau-basalt forming
portions of the Neuweld range and the elevated table-land of Cape
Colony, may be regarded as forerunners of those just described, and
possibly contemporaneous with the Ashangi volcanic series of Abyssinia.
The great basaltic sheets of the Cape Colony are found capping the
highest elevations of the Camderboo and Stormberg ranges, as well as
overspreading immense areas of less elevated land, to an extent,
according to Professor A. H. Green, of at least 120,000 square miles.[1]
Amongst these sheets, innumerable dykes, and masses of solid lava which
filled the old vents of eruption, are to be observed. The floor upon
which the lava-floods have been poured out generally consists of the
"Cave Sandstone," the uppermost of a series of deposits which had
previously been laid down over the bed of an extensive lake which
occupied this part of Africa during the Mesozoic period. After the
deposition of this sandstone, the volcanic forces appear to have burst
through the crust, and from vents and fissures great floods of augitic
lava, with beds of tuff, invaded the region occupied by the waters of
the lake. The lava-sheets have since undergone extensive denudation, and
are intersected by valleys and depressions eroded down through them into
the sandstone floor beneath; and though the precise geological period at
which they were extruded must remain in doubt, it appears probable that
they may be referred to that of the Trias.[2]
[1] Green. "On the Geology of the Cape Colony," _Quart. Jour. Geol.
Soc._, vol. xliv. (1888).
[2] The district lying along the south coast of Africa is described by
Andrew G. Bain, in the _Trans. Geol. Soc._, vol. vii. (1845); but there
is little information regarding the volcanic region here referred to.
CHAPTER IV.
VOLCANIC ROCKS OF PAST GEOLOGICAL PERIODS OF THE BRITISH ISLES.
It is beyond the scope of this work to describe the volcanic rocks of
pre-Tertiary times over various parts of the globe. The subject is far
too large to be treated otherwise than in a distinct and separate essay.
I will therefore content myself with a brief enumeration of the
formations of the British Isles in which contemporaneous volcanic action
has been recognised.[1]
There is little evidence of volcanic action throughout the long lapse of
time extending backwards from the Cretaceous to the Triassic epochs,
that is to say, throughout the Mesozoic or Secondary period, and it is
not till we reach the Palaeozoic strata that evidence of volcanic action
unmistakably presents itself.
_Permian Period._--In Ayrshire, and in the western parts of Devonshire,
beds of felspathic porphyry, felstone and ash are interstratified with
strata believed to be of Permian age. In Devonshire these have only
recently been recognised by Dr. Irving and the author as of Permian age,
the strata consisting of beds of breccia, lying at the base of the New
Red Sandstone. Those of Ayrshire have long been recognised as of the
same period; as they rest unconformably on the coal measures, and
consist of porphyrites, melaphyres, and tuffs of volcanic origin.
_Carboniferous Period._--Volcanic rocks occur amongst the coal-measures
of England and Scotland, while they are also found interbedded with the
Carboniferous Limestone series in Derbyshire, Scotland, and Co. Limerick
in Ireland. The rocks consist chiefly of basalt, dolerite, melaphyre and
felstone.
_Devonian Period._--Volcanic rocks of Devonian age occur in the South of
Scotland, consisting of felstone-porphyries and melaphyres; also at
Boyle, in Roscommon, and amongst the Glengariff beds near Killarney in
Ireland.
_Upper Silurian Period._--Volcanic rocks of this stage are only known in
Ireland, on the borders of Cos. Mayo and Galway, west of Lough Mask, and
at the extreme headland of the Dingle Promontory in Co. Kerry. They
consist of porphyrites, felstones and tuffs, or breccias,
contemporaneously erupted during the Wenlock and Ludlow stages. Around
the flanks of Muilrea, beds of purple quartz-felstone with tuff are
interstratified with the Upper Silurian grits and slates.
_Lower Silurian Period._--Volcanic action was developed on a grand scale
during the Arenig and Caradoc-Bala stages, both in Wales and the Lake
district, and in the Llandeilo stage in the South of Scotland. The
felspathic lavas, with their associated beds of tuff and breccia, rise
into some of the grandest mountain crests of North Wales, such as those
of Cader Idris, Aran Mowddwy, Arenig and Moel Wyn. A similar series is
also represented in Ireland, ranging from Wicklow to Waterford, forming
a double group of felstones, porphyries, breccias, and ash-beds, with
dykes of basalt and dolerite. The same series again appears amidst the
Lower Silurian beds of Co. Louth, near Drogheda.
_Metamorphic Series presumably of Lower Silurian Age._--If, as seems
highly probable, the great metamorphic series of Donegal and Derry are
the representatives in time of the Lower Silurian series, some of the
great sheets of felspathic and hornblendic trap which they contain are
referable to this epoch. These rocks have undergone a change in
structure along with the sedimentary strata of which they were
originally formed, so that the sheets of (presumably) augitic lava have
been converted into hornblende-rock and schist. Similar masses occur in
North Mayo, south of Belderg Harbour.
_Cambrian Period._--In the Pass of Llanberis, along the banks of Llyn
Padarn, masses of quartz-porphyry, felsite and agglomerate, or breccia,
indicate volcanic action during this stage. These rocks underlie beds of
conglomerate, slate and grit of the Lower Cambrian epoch, and, as Mr.
Blake has shown, are clearly of volcanic origin, and pass upwards into
the sedimentary strata of the period. A similar group, first recognised
by Professor Sedgwick, stretches southwards from Bangor along the
southern shore of the Menai Straits. Again, we find the volcanic
eruptions of this epoch at St. David's, consisting of diabasic and
felsitic lava, with beds of ash; and in the centre of England, amongst
the grits and slates of Charnwood Forest presumably of Cambrian age,
various felstones, porphyries, and volcanic breccias are found.
Thus it will be seen that every epoch, from the earliest stage of the
Cambrian to the Permian, in the British Isles, gives evidence of the
existence of volcanic action; from which we may infer that the
originating cause, whatever it may be, has been in operation throughout
all past geological time represented by living forms. The question of
the condition of our globe in Archaean times, and earlier, is one which
only can be discussed on theoretic ground, and is beyond the scope of
this work.
[1] The reader is referred to Sir A. Geikie's Presidential Address to
the Geological Society (1891) for the latest view of this subject.
[Illustration: VOLCANIC BAND OF THE MOLUCCAS.
Map showing the volcanic belt to which Krakatoa belongs. The shaded
portion is volcanic.]
PART VI.
SPECIAL VOLCANIC AND SEISMIC PHENOMENA.
CHAPTER I.
THE ERUPTION OF KRAKATOA IN 1883.
I propose to introduce here some account of one of the most terrible
outbursts of volcanic action that have taken place in modern times;
namely, the eruption of the volcano of Krakatoa (a corruption of Rakata)
in the strait of Sunda, between the islands of Sumatra and Java, in the
year 1883. The Malay Archipelago, of which this island once formed a
member, is a region where volcanic action is constant, and where the
outbursts are exceptionally violent. With the great island of Borneo as
a solid, non-volcanic central core, a line of volcanic islands extends
from Chedooba off the coast of Pegu through Sumatra, Java, Sumbawa,
Flores, and, reaching the Moluccas, stretches northwards through the
Philippines into Japan and Kamtschatka. This is probably the most active
volcanic belt in the world, and the recent terrible earthquake and
eruption in Japan (November, 1891) gives proof that the volcanic forces
are as powerful and destructive as ever.[1]
(_a._) _Dormant Condition down to 1680._--Down to the year 1680, this
island, although from its form and structure evidently volcanic, appears
to have been in a dormant state; its sides were covered with luxuriant
forests, and numerous habitations dotted its shore. But in May of that
year an eruption occurred, owing to which the aspect of Krakatoa as
described by Vogel was entirely changed; the surface of the island when
this writer passed on his voyage to Sumatra appeared burnt up and arid,
while blocks of incandescent rock were being hurled into the air from
four distinct points. After this first recorded eruption the island
relapsed into a state of repose, and except for a stream of molten lava
which issued from the northern extremity, there was no evidence of its
dangerous condition. The luxuriant vegetation of the tropics speedily
re-established itself, and the volcano was generally regarded as
"extinct."[2] History repeats itself; and the history of Vesuvius was
repeated in the case of Krakatoa.
[Illustration: Fig. 35.--Map Of The Krakatoa Group Of Islands Before
The Eruption Of August 1883 (From Admiralty Chart)]
(_b._) _Eruption of May, 1883._[3]--On the morning of May 20, 1883, the
inhabitants of Batavia, of Buitenzorg, and neighbouring localities, were
surprised by a confused noise, mingled with detonations resembling the
firing of artillery. The phenomena commenced between ten and eleven
o'clock in the morning, and soon acquired such intensity as to cause
general alarm. The detonations were accompanied by tremblings of the
ground, of buildings and various objects contained in dwellings; but it
was generally admitted that these did not proceed from earthquake
shocks, but from atmospheric vibrations. No deviation of the magnetic
needle was observed at the Meteorological Institute of Batavia; but a
vertical oscillation was apparent, and persons who listened with the ear
placed on the ground, even during the most violent detonations, could
hear no subterranean noise whatever. It became clear that the sounds
came from some volcano burst into activity; but it is strange that for
two whole days it remained uncertain what was the particular volcano to
which the phenomena were to be referred. The detonations appeared,
indeed, to come from the direction of Krakatoa; but from Serang, Anjer,
and Merak, localities situated much nearer Krakatoa than Batavia, the
telegraph announced that neither detonations nor atmospheric vibrations
had been perceived. The distance between Batavia and Krakatoa is
ninety-three English miles. The doubts thus experienced were, however,
soon put to rest by the arrival of an American vessel under the command
of A. R. Thomas, and of other ships which hailed from the straits of
Sunda. From their accounts it was ascertained that in the direction of
Krakatoa the heavens were clouded with ashes, and that a grand column of
smoke, illumined from time to time by flashes of flame, arose from above
the island. Thus after a repose of more than two hundred years, "the
peaceable isle of Krakatoa, inhabited, and covered by thick forests, was
suddenly awakened from its condition of fancied security."
[Illustration: Fig. 36.--Section from Verlaten Island through Krakatoa,
to show the outline before and after the eruption of August, 1888. The
continuous line shows the former; the dotted line and shading, the
latter; from which it will be observed that the original island has to a
large extent disappeared. The line of section is shown in Fig. 35.]
(_c._) _Form and Appearance of the Island before the Eruption of
1883._--From surveys made in 1849 and 1881, it would appear that the
island of Krakatoa consisted of three mountains or groups of mountains
(Figs. 35, 36); the southern formed by the cone of Rakata (properly so
called), rising with a scarped face above the sea to a height of over
800 metres (2,622 feet). Adjoining this cone, and rising from the centre
of the island, came the group of Danan, composed of many summits,
probably forming part of the _enceinte annulaire_ of a crater. And near
the northern extremity of the isle, a third group of mammelated heights
could be recognised under the general name of Perboewatan, from which
issued several obsidian lava-flows, with a steep slope; these dated back
perhaps to the period of the first known eruption of 1680. This large
and mountainous island as it existed at the beginning of May, 1883, has
been entirely destroyed by the terrible eruptions of that year, with the
exception of the peripheric rim (composed of the most ancient of the
volcanic rocks, andesite), of which Verlaten Island and Rakata formed a
part, and one very small islet, which is noted on the maps as "rots"
(rock), and on the new map of the Straits of Sunda of the Dutch Navy as
that of "Bootsmansrots."[4]
As shown by the map in the Report of the Royal Society, the group of
islands which existed previous to 1883 were but the unsubmerged portions
of one vast volcanic crater, built up of a remarkable variety of lava
allied to the andesite of the Java volcanoes, but having a larger
percentage of silica, and hence falling under the head of
"enstatite-dacite."[5] That these volcanic rocks are of very recent
origin is shown by the fact, ascertained by Verbeek, that beneath them
occur deposits of Post-Tertiary age, and that these in turn rest on the
Tertiary strata which are widely distributed through Sumatra, Java, and
the adjoining islands. According to the reasoning of Professor Judd, the
Krakatoa group at an early period of its history presented the form of a
magnificent crater-cone, several miles in circumference at the base,
which subsequent eruptions shattered into fragments or blew into the
air in the form of dust, ashes, and blocks of lava, while the central
part collapsed and fell in, leaving a vast circular ring like the
ancient crater of Somma (see Fig. 6, p. 43), and he supposes the former
eruptions to have been on a scale exceeding in magnificence those which
have caused such world-wide interest within the last few years.
(_d._) _Eruption of 26th to 28th of August._--It was, as we have seen,
in the month of May that, in the language of Chev. Verbeek, "the volcano
of Krakatoa chose to announce in a high voice to the inhabitants of the
Archipelago that, although almost nothing amongst the many colossal
volcanic mountains of the Indies, it yielded to none of them in regard
to its power." These eruptions were, however, only premonitory of the
tremendous and terrible explosion which was to commence on Sunday, the
26th of August, and which continued for several days subsequently. A
little after noon of that day, a rumbling noise accompanied by short and
feeble explosions was heard at Buitenzorg, coming from the direction of
Krakatoa; and similar sounds were heard at Anjer and Batavia a little
later. Soon these detonations augmented in intensity, especially about
five o'clock in the evening; and news was afterwards received that the
sounds had been heard in the isle of Java. These sounds increased still
more during the night, so that few persons living on the west side of
the isle of Java were able to sleep. At seven in the morning there came
a crash so formidable, that those who had hoped for a little sleep at
Buitenzorg leaped from their beds. Meanwhile the sky, which had up to
this time been clear, became overcast, so that by ten o'clock it became
necessary to have recourse to lamps, and the air became charged with
vapour. Occasional shocks of earthquake were now felt. Darkness became
general all over the straits and the bordering coasts. Showers of ashes
began to fall. The repeated shocks of earthquake, and the rapid
discharges of subterranean artillery, all combined to show that an
eruption of even greater violence than that of May was in progress at
the isle of Krakatoa.
But the most interested witnesses to this terrible outburst were those
on board the ships plying through the straits. Amongst these was the
_Charles Bal_, a British vessel under the command of Captain Watson.
This ship was ten miles south of the volcano on Sunday afternoon, and
therefore well in sight of the island at the time when the volcano had
entered upon its paroxysmal state of action. Captain Watson describes
the island as being covered by a dense black cloud, while sounds like
the discharges of artillery occurred at intervals of a second of time;
and a crackling noise (probably arising from the impact of fragments of
rock ascending and descending in the atmosphere) was heard by those on
board. These appearances became so threatening towards five o'clock in
the evening, that the commander feared to continue his voyage and began
to shorten sail. From five to six o'clock a rain of pumice in large
pieces, quite warm, fell upon the ship, which was one of those that
escaped destruction during this terrible night.[6]
(_e._) _Electrical Phenomena._--During this eruption, electrical
phenomena of great splendour were observed. Captain Wooldbridge,
viewing the eruption in the afternoon of the 26th from a distance of
forty miles, speaks of a great vapour-cloud looking like an immense wall
being momentarily lighted up "by bursts of forked lightning like large
serpents rushing through the air. After sunset this dark wall resembled
a blood-red curtain, with edges of all shades of yellow, the whole of a
murky tinge, through which gleamed fierce flashes of lightning." As
Professor Judd observes, the abundant generation of atmospheric
electricity is a familiar phenomenon in all volcanic eruptions on a
grand scale. The steam-jets rushing through the orifices of the earth's
crust constitute an enormous hydro-electrical engine, and the friction
of the ejected materials striking against one another in their ascent
and descent also does much in the way of generating electricity.[7] It
has been estimated by several observers that the column of watery vapour
ascended to a height of from twelve to seventeen and even twenty-three
miles; and on reaching the upper strata of the atmosphere, it spread
itself out in a vast canopy resembling "the pine-tree" form of Vesuvian
eruptions; and throughout the long night of the 27th this canopy
continued to extend laterally, and the particles of dust which it
enclosed began to descend slowly through the air.
(_f._) _Formation of Waves._--This tremendous outburst of volcanic
forces, which to a greater or less extent influenced the entire surface
of the globe, gave rise to waves which traversed both air and ocean; and
in consequence of the large number of observatories scattered all over
the globe, and the excellence and delicacy of the instruments of
observation, we are put in possession of the remarkable results which
have been obtained from the collection of the observations in the hands
of competent specialists. The results are related _in extenso_ in the
Report of the Royal Society, illustrated by maps and diagrams, and are
worthy of careful study by those interested in terrestrial phenomena. A
brief summary is all that can be given here, but it will probably
suffice to bring home to the reader the magnitude and grandeur of the
eruption.
The vibrations or waves generated in August, 1883, at Krakatoa may be
arranged under three heads: (1) Atmospheric Waves; (2) Sound Waves; and
(3) Oceanic Waves; which I will touch upon in the order here stated.
(1) _Atmospheric Waves._--These phenomena have been ably handled by
General Strachey,[8] from a large number of observations extending all
over the globe. From these it has been clearly established that an
atmospheric wave, originating at Krakatoa as a centre, expanded outwards
in a circular form and travelled onwards till it became a great circle
at a distance of 180 degrees from its point of origin, after which it
still advanced, but now gradually contracting to a node at the antipodes
of Krakatoa; that is to say, at a point over the surface of North
America, situated in lat. 6 deg. N. and long. 72 deg. W. (or thereabout). Having
attained this position, the wave was reflected or reproduced, expanding
outwards for 180 degrees and travelling backwards again to Krakatoa,
from which it again started, and returning to its original form again
overspread the globe. This wonderful repetition, due to the spherical
form of the earth, was observed no fewer than seven times, though with
such diminished force as ultimately to be outside the range of
observation by the most sensitive instruments. It is one of the triumphs
of modern scientific appliances that the course of such a wave,
generated in a fluid surrounding a globe, which might be demonstrated on
mathematical principles, has been actually determined by experiments
carried on over so great an area.
(2) _Sound Waves._--If the sound-waves produced at the time of maximum
eruption were not quite as far-reaching as those of the air, they were
certainly sufficiently surprising to be almost incredible, were it not
that they rest, both as regards time and character, upon incontestible
authority. The sound of the eruption, resembling that of the discharge
of artillery, was heard not only over nearly all parts of Sumatra, Java,
and the coast of Borneo opposite the Straits of Sunda, but at places
over two thousand miles distant from the scene of the explosions.
Detailed accounts, collected with great care, are given in the Report of
the Royal Society, from which the following are selected as examples:--
1. At the port of Acheen, at the northern extremity of Sumatra,
distant 1,073 miles, it was supposed that the port was being
attacked, and the troops were put under arms.
2. At Singapore, distant 522 miles, two steamers were dispatched to
look out for the vessel which was supposed to be firing guns as
signals of distress.
3. At Bankok, in Siam, distant 1,413 miles, the report was heard on
the 27th; as also at Labuan, in Borneo, distant 1,037 miles.
4. At places in the Philippine Islands, distant about 1,450 miles,
detonations were heard on the 27th, at the time of the eruption.
The above places lie northwards of Krakatoa. In the opposite direction,
we have the following examples:--
5. At Perth, in Western Australia, distant 1,092 miles, sounds as
of guns firing at sea were heard; and at the Victorian Plains,
distant about 1,700 miles, similar sounds were heard.
6. In South Australia, at Alice's Springs, Undoolga, and other
places at distances of over 2,000 miles, the sounds of the eruption
were also heard.
7. In a westerly direction at Dutch Bay, Ceylon, distant 2,058
miles, the sounds were heard between 7 a.m. and 10 a.m. on the
morning of the 27th of August.
8. Lastly, at the Chagos Islands, distant 2,267 miles, the
detonations were audible between 10 and 11 a.m. of the same day.
Some of the above distances are so great that we may fail to realise
them; but they will be more easily appreciated, perhaps, if we change
the localities to our own side of the globe, and take two or three cases
with similar distances. Then, if the eruption had taken place amongst
the volcanoes of the Canaries, the detonations would have been heard at
Gibraltar, at Lisbon, at Portsmouth, Southampton, Cork, and probably at
Dublin and Liverpool; or, again, supposing the eruption had taken place
on the coast of Iceland, the report would have been heard all over the
western and northern coasts of the British Isles, as well as at
Amsterdam and the Hague. The enormous distance to which the sound
travelled in the case of Krakatoa was greatly due to the fact that the
explosions took place at the surface of the sea, and the sound was
carried along that surface uninterruptedly to the localities recorded; a
range of mountains intervening would have cut off the sound-wave at a
comparatively short distance from its source.
(3) _Oceanic Waves._--As may be supposed, the eruption gave rise to
great agitation of the ocean waters with various degrees of vertical
oscillation; but according to the conclusions of Captain Wharton,
founded on numerous data, the greatest wave seems to have originated at
Krakatoa about 10 a.m. on the 27th of August, rising on the coasts of
the Straits of Sunda to a height of fifty feet above the ordinary
sea-level. This wave appears to have been observed over at least half
the globe. It travelled westwards to the coast of Hindostan and Southern
Arabia, ultimately reaching the coasts of France and England. Eastwards
it struck the coast of Australia, New Zealand, the Sandwich Islands,
Alaska, and the western coast of North America; so that it was only the
continent of North and South America which formed a barrier (and that
not absolute) to the circulation of this oceanic wave all over the
globe. The destruction to life and property caused by this wave along
the coasts of Sunda was very great. Combined with the earthquake shocks
(which, however, were not very severe), the tremendous storm of wind,
the fall of ashes and cinders, and the changes in the sea-bed, it
produced in the Straits of Sunda for some time after the eruption a
disastrous transformation. Lighthouses had been swept away; all the old
familiar landmarks on the shore were obscured by a vast deposit of
volcanic dust; the sea itself was encumbered with enormous quantities of
floating pumice, in many places of such thickness that no vessel could
force its way through them; and for months after the eruption one of the
principal channels was greatly obstructed by two islands which had
arisen in its midst. The Sebesi channel was completely blocked by banks
composed of volcanic materials, and two portions of these banks rose
above the sea as islands, which received the name of "Steers Island" and
"Calmeyer Island"; but these, by the action of the waves, have since
been completely swept away, and the materials strewn over the bed of the
sea.[9]
(_g._) _Atmospheric Effects._--But the face of nature, even in her most
terrific and repulsive aspect, is seldom altogether unrelieved by some
traces of beauty. In contrast to the fearful and disastrous phenomena
just described, is to be placed the splendour of the heavens, witnessed
all over the central regions of the globe throughout a period of several
months after the eruption of 1883, which has been ably treated by the
Hon. Rollo Russell and Mr. C. D. Archibald, in the Royal Society's
Report.
When the particles of lava and ashes mingled with vapour were projected
into the air with a velocity greater than that of a ball discharged from
the largest Armstrong gun, these materials were carried by the prevalent
trade-winds in a westerly direction, and some of them fell on the deck
of ships sailing in the Indian Ocean as far as long. 80 deg. E., as in the
case of the _British Empire_--on which the particles fell on the 29th of
August, at a distance of 1,600 miles from Krakatoa. But far beyond this
limit, the finer particles of dust (or rather minute crystals of felspar
and other minerals), mingled with vapour of water, were carried by the
higher currents of the air as far as the Seychelles and Africa,--not
only the East coast, but also the West, as Cape Coast Castle on the Gold
Coast; to Paramaribo, Trinidad, Panama, the Sandwich Isles, Ceylon and
British India, at all of which places during the month of September the
sun assumed tints of blue or green, as did also the moon just before and
after the appearance of the stars;[10] and from the latter end of
September and for several months, the sky was remarkable for its
magnificent coloration; passing from crimson through purple to yellow,
and melting away in azure tints which were visible in Europe and the
British Isles; while a large corona was observed round both the sun and
moon. These beautiful sky effects were objects of general observation
throughout the latter part of the year 1883 and commencement of the
following year.
The explanation of these phenomena may be briefly stated. The fine
particles, consisting for the most part of translucent crystals, or
fragments of crystals, formed a canopy high up in the atmosphere, being
gradually spread over both sides of the equator till it formed a broad
belt, through which the rays of the sun and moon were refracted. Towards
dawn and sunset they were refracted and reflected from the facets of the
crystal, and thus underwent decomposition into the prismatic colours; as
do the rays of the sun when refracted and reflected from the particles
of moisture in a rain-cloud. The subject is one which cannot be fully
dealt with here, and is rather outside the scope of this work.
(_h._) _Origin of the Eruption._--The ultimate cause of volcanic
eruptions is treated in a subsequent chapter, nor is that of Krakatoa
essentially different from others. It was remarkable, however, both for
the magnitude of the forces evoked and the stupendous scale of the
resulting phenomena. It is evident that water played an important part
in these phenomena, though not as the prime mover;--any more than water
in the boiler of a locomotive is the prime mover in the generation of
the steam. Without the fuel in the furnace the steam would not be
produced; and the amount of steam generated will be proportional to the
quantity and heat of the fuel in the furnace and the quantity of water
in the boiler. In the case of Krakatoa, both these elements were
enormous and inexhaustible. The volcanic chimney (or system of
chimneys), being situated on an island, was readily accessible to the
waters of the ocean when fissures gave them access to the interior
molten matter. That such fissures were opened we may well believe. The
earthquakes which occurred at the beginning of May, and later on, on the
27th of that month, may indicate movements of the crust by which water
gained access. It appears that in May the only crater in a state of
activity was that of Perboewatan; in June another crater came into
action, connected with Danan in the centre of the island, and in August
a third burst forth. Thus there was progressive activity up to the
commencement of the grand eruption of the 26th of that month.[11] During
this last paroxysmal stage, the centre of the island gave way and sunk
down, when the waters of the ocean gained free access, and meeting with
the columns of molten matter rising from below originated the prodigious
masses of steam which rose into the air.
(_i._) _Cause of the Detonations._--The detonations which accompanied
the last great eruption are repeatedly referred to in all the accounts.
These may have been due, not only to the sudden explosions of steam
directly produced by the ocean water coming in contact with the molten
lava, but by dissociation of the vapour of water at the critical point
of temperature into its elements of oxygen and hydrogen; the reunion of
these elements at the required temperature would also result in
explosions.
The phenomena attending this great volcanic eruption, so carefully
tabulated and critically examined, will henceforth be referred to as
constituting an epoch in the history of volcanic action over the globe,
and be of immense value for reference and comparison.
[1] The eruption of Krakatoa has been the subject of an elaborate Report
published by the Royal Society, and is also described in a work by
Chevalier R. D. M. Verbeek, Ingenieur en Chef des Mines, and published
by order of the Governor-General of the Netherland Indies (1886). See
also an Article by Sir R. S. Ball in the _Contemporary Review_ for
November, 1888.
[2] Verbeek, _loc. cit._, p. 4.
[3] The account of this eruption is a free translation from Verbeek.
[4] Verbeek, _loc. cit._, p. 160.
[5] Judd, _Rep. R. S._
[6] A fuller account by Prof. Judd will be found in the _Report of the
Royal Society_, p. 14. Several vessels at anchor were driven ashore on
the straits owing to the strong wind which arose.
[7] Judd, _Report_, p. 21.
[8] _Report_, Part ii.
[9] In this eruption, 36,380 human beings perished, of whom 37 were
Europeans; 163 villages (_kampoengs_) were entirely, and 132 partially,
destroyed.--Verbeek, _loc. cit._, p. 79.
[10] Verbeek, _loc. cit._, p. 144-5. The dust put a girdle round the
earth in thirteen days.
[11] Verbeek, _loc. cit._, p. 30.
CHAPTER II.
EARTHQUAKES.
_Connection of Earthquakes with Volcanic Action._--The connection
between earthquake shocks and volcanic eruptions is now so generally
recognised that it is unnecessary to insist upon it here. All volcanic
districts over the globe are specially liable to vibrations of the
crust; but at the same time it is to be recollected that these movements
visit countries occasionally from which volcanoes, either recent or
extinct, are absent; in which cases we may consider earthquake shocks to
be abortive attempts to originate volcanic action.
(_a._) _Origin._--From the numerous observations which have been made
regarding the nature of these phenomena by Hopkins, Lyell, and others,
it seems clearly established that earthquakes have their origin in some
sudden impact of gas, steam, or molten matter impelled by gas or steam
under high pressure, beneath the solid crust.[1] How such impact
originates we need not stop to inquire, as the cause is closely
connected with that which produces volcanic eruptions. The effect,
however, of such impact is to originate a wave of translation through
the crust, travelling outwards from the point, or focus, on the surface
immediately over the point of impact.[2] These waves of translation can
in some cases be laid down on a map, and are called "isoseismal curves,"
each curve representing approximately an equal degree of seismal
intensity; as shown on the chart of a part of North America affected by
the great Charleston earthquake. (Fig. 37.) Mr. Hopkins has shown that
the earthquake-wave, when it passes through rocks differing in density
and elasticity, changes in some degree not only its velocity, but its
direction; being both refracted and reflected in a manner analogous to
that of light when it passes from one medium to another of different
density.[3] When a shock traverses the crust through a thickness of
several miles it will meet with various kinds of rock as well as with
fissures and plications of the strata, owing to which its course will be
more or less modified.
(_b._) _Formation of Fissures._--During earthquake movements, fissures
may be formed in the crust, and filled with gaseous or melted matter
which may not in all cases reach the surface; and, on the principle that
volcanoes are safety-valves for regions beyond their immediate
influence, we may infer that the earthquake shock, which generally
precedes the outburst of a volcano long dormant, finds relief by the
eruption which follows; so that whatever may be the extent of the
disastrous results of such an eruption, they would be still more
disastrous if there had been no such safety-valve as that afforded by a
volcanic vent. Thus, probably, owing to the extinction of volcanic
activity in Syria, the earthquakes in that region have been peculiarly
destructive. For example, on January 1, 1837, the town of Safed west of
the Jordan valley was completely destroyed by an earthquake in which
most of the inhabitants perished. The great earthquakes of Aleppo in the
present century, and of Syria in the middle of the eighteenth, were of
exceptional severity. In that of Syria, which took place in 1759, and
which was protracted during a period of three months, an area of 10,000
square leagues was affected. Accon, Saphat, Baalbeck, Damascus, Sidon,
Tripoli, and other places were almost entirely levelled to the ground;
many thousands of human beings lost their lives.[4] Other examples might
be cited.
(_c._) _Earthquake Waves._--We have now to return to the phenomena
connected with the transmission of earthquake-waves. The velocity of
transmission through the earth is very great, and several attempts have
been made to measure this velocity with accuracy. The most valuable of
such attempts are those connected with the Charleston and Riviera
shocks. Fortunately, owing to the perfection of modern appliances, and
the number of observers all over the globe, these results are entitled
to great confidence. The phenomena connected with the Charleston
earthquake, which took place on the 31st of August, 1886, are described
in great detail by Captain Clarence E. Dutton, of the U.S. Ordnance
Corps.[5] The conclusions arrived at are;--that as regards the depth of
the focal point, this is estimated at twelve miles, with a probable
error of less than two miles; while, as regards the rate of travel of
the earthquake-wave, the estimate is (in one case) about 3.236 miles per
second; and in another about 3.226 miles per second.
On the other hand, in the case of the earthquake of the Riviera, which
took place on the 23rd of February, 1887, at 5.30 a.m. (local time), the
vibrations of which appear to have extended across the Atlantic, and to
have sensibly affected the seismograph in the Government Signal Office
at Washington, the rate of travel was calculated at about 500 miles per
hour, less than one-half that determined in the case of Charleston; but
Captain Dutton claims, and probably with justice, that the results
obtained in the latter case are far more reliable than any hitherto
arrived at.
(_d._) _Oceanic Waves._--When the originating impact takes place under
the bed of the ocean--either by a sudden up-thrust of the crust to the
extent, let us suppose, of two or three feet, or by an explosion from a
submarine volcano--a double wave is formed, one travelling through the
crust, the other through the ocean; and as the rate of velocity of the
former is greatly in excess of that of the latter, the results on their
reaching the land are often disastrous in the extreme. It is the
ocean-wave, however, which is the more important, and calls for special
consideration. If the impact takes place in very deep water, the whole
mass of the water is raised in the form of a low dome, sloping equally
away in all directions; and it commences to travel outwards as a wave
with an advancing crest until it approaches the coast and enters shallow
water. The wave then increases in height, and the water in front is
drawn in and relatively lowered; so that on reaching a coast with a
shelving shore the form of the surface consists of a trough in front
followed by an advancing crest. These effects may be observed on a small
scale in the case of a steamship advancing up a river, or into a harbour
with a narrow channel, but are inappreciable in deep water, or along a
precipitous open coast.
(_e._) _The Earthquake of Lisbon, 1755._--The disastrous results of a
submarine earthquake upon the coast have never been more terribly
illustrated than in the case of the earthquake of Lisbon which took
place on November 1, 1755. The inhabitants had no warning of the coming
danger, when a sound like that of thunder was heard underground, and
immediately afterwards a violent shock threw down the greater part of
their city; this was the land-wave. In the course of about six minutes,
sixty thousand persons perished. The sea first retired and left the
harbour dry, so forming the trough in front of the crest; immediately
after the water rolled in with a lofty crest, some 50 feet above the
ordinary level, flooding the harbour and portions of the city bordering
the shore. The mountains of Arrabida, Estrella, Julio, Marvan, and
Cintra, were impetuously shaken, as it were, from their very
foundations; and according to the computation of Humboldt, a portion of
the earth's surface four times the extent of Europe felt the effects of
this great seismic shock, which extended to the Alps, the shores of the
Baltic, the lakes of Scotland, the great lakes of North America, and
the West Indian Islands. The velocity of the sea-wave was estimated at
about 20 miles per minute.
(_f._) _Earthquake of Lima and Callao, 28th October, 1746._--Of somewhat
similar character was the terrible catastrophe with which the cities of
Lima and Callao were visited in the middle of the last century,[6] in
which the former city, then one of great magnificence, was overthrown;
and Callao was inundated by a sea-wave, in which out of 23 ships of all
sizes in the harbour the greater number foundered; several, including a
man-of-war, were lifted bodily and stranded, and all the inhabitants
with the exception of about two hundred were drowned. A volcano in
Lucanas burst forth the same night, and such quantities of water
descended from the cone that the whole country was overflowed; and in
the mountain near Pataz, called Conversiones de Caxamarquilla, three
other volcanoes burst forth, and torrents of water swept down their
sides. In the case of these cities, the land-wave, or shock, preceded
the sea-wave, which of course only reached the port of Callao.
[Illustration: Fig. 37.--The lines represent isoseismal curves, or
curves of equal intensity, the force decreasing outwards from the focus
at Charleston, No. 10.]
(_g._) _Earthquake of Charleston, 31st August, 1886._--I shall close
this account of some remarkable earthquakes with a few facts regarding
that of Charleston, on the Atlantic seaboard of Carolina.[7] At 9.51
a.m. of this day, the inhabitants engaged in their ordinary occupations
were startled by the sound of a distant roar, which speedily deepened in
volume so as to resemble the noise of cannon rattling along the road,
"spreading into an awful noise, that seemed to pervade at once the
troubled earth below and the still air above." At the same time the
floors began to heave underfoot, the walls visibly swayed to and fro,
and the crash of falling masonry was heard on all sides, while
universal terror took possession of the populace, who rushed into the
streets, the black portion of the community being the most demonstrative
of their terror. Such was the commencement of the earthquake, by which
nearly all the houses of Charleston were damaged or destroyed, many of
the public buildings seriously injured or partially demolished. The
effects were felt all over the States as far as the great lakes of
Canada and the borders of the Rocky Mountains. Two epicentral _foci_
appear to have been established; one lying about 15 miles to the N.W. of
Charleston, called the _Woodstock focus_; the other about 14 miles due
west of Charleston, called the _Rantowles focus_; around each of these
_foci_ the isoseismic curves concentrated,[8] but in the map (Fig. 37)
are combined into the area of one curve. The position of these _foci_
clearly shows that the origin of the Charleston earthquake was not
submarine, though occurring within a short distance of the Atlantic
border; the curves of equal intensity (isoseismals) are drawn all over
the area influenced by the shock.
As a general result of these detailed observations, Captain Dutton
states that there is a remarkable coincidence in the phenomena with
those indicated by the theory of wave-motion as the proper one for an
elastic, nearly homogeneous, solid medium, composed of such materials as
we know to constitute the rocks of the outer portions of the earth; but
on the other hand he states that nothing has been disclosed which seems
to bring us any nearer to the precise nature of the forces which
generated the disturbance.[9]
[1] The views of Mr. R. Mallet, briefly stated, are somewhat as
follows:--Owing to the secular cooling of the earth, and the consequent
lateral crushing of the surface, this crushing from time to time
overcomes the resistance; in which case shocks are experienced along the
lines of fracture and faulting by which the crust is intersected. These
shocks give rise to earthquake waves, and as the crushing of the walls
of the fissure developes heat, we have here the _vera causa_ both of
volcanic eruptions and earthquake shocks--the former intensified into
explosions by access of water through the fissures.--"On the Dynamics of
Earthquakes," _Trans. Roy. Irish Acad._, vol. xxi.
[2] Illustration of the mode of propagation of earthquake shocks will be
found in Lyell's _Principles of Geology_, vol. ii. p. 136, or in the
author's _Physiography_, p. 76, after Hopkins.
[3] "Rep. on Theories of Elevation and Earthquakes," _Brit. Ass. Rep._
1847, p. 33. In the map prepared by Prof. J. Milne and Mr. W. K. Burton
to show the range of the great earthquake of Japan (1891), similar
isoseismal lines are laid down.
[4] Lyell, _loc. cit._, p. 163. Two Catalogues of Earthquakes have been
drawn up by Prof. O'Reilly, and are published in the _Trans. Roy. Irish
Academy_, vol. xxviii. (1884 and 1886).
[5] _Ninth Annual Report, U.S. Geological Survey_ (1888).
[6] _A True and Particular Account of the Dreadful Earthquake_, 2nd
edit. The original published at Lima by command of the Viceroy. London,
1748. Translated from the Spanish.
[7] I take the account from that of Capt. Dutton above cited, p. 220.
[8] Dutton, _Report_, Plate xxvi., p. 308.
[9] _Ibid._, p. 211. On the connection between the moon's position and
earthquake shocks, see Mr. Richardson's paper on Scottish earthquakes,
_Trans. Edin. Geol. Soc._, vol. vi. p. 194 (1892).
PART VII.
VOLCANIC AND SEISMIC PROBLEMS.
CHAPTER I.
THE ULTIMATE CAUSE OF VOLCANIC ACTION.
Volcanic phenomena are the outward manifestations of forces deep-seated
beneath the crust of the globe; and in seeking for the causes of such
phenomena we must be guided by observation of their nature and mode of
action. The universality of these phenomena all over the surface of our
globe, in past or present times, indicates the existence of a general
cause beneath the crust. It is true that there are to be found large
tracts from which volcanic rocks (except those of great geological
antiquity) are absent, such as Central Russia, the Nubian Desert, and
the Central States of North America; but such absence by no means
implies the non-existence of the forces which give rise to volcanic
action beneath those regions, but only that the forces have not been
sufficiently powerful to overcome the resistance offered by the crust
over those particular tracts. On the other hand, the similarity of
volcanic lavas over wide regions is strong evidence that they are drawn
from one continuous magma, consisting of molten matter beneath the
solid exterior crust.
(_a._) _Lines of Volcanic Action._--It has been shown in a previous page
that volcanic action of recent or Tertiary times has taken place mainly
along certain lines which may be traced on the surface of a map or
globe. One of these lines girdles the whole globe, while others lie in
certain directions more or less coincident with lines of flexure,
plication or faulting. The Isle of Sumatra offers a remarkable example
of the coincidence of such lines with those of volcanic vents. Not only
the great volcanic cones, but also the smaller ones, are disposed in
chains which run parallel to the longitudinal axis of the island
(N.W.-S.E.). The sedimentary rocks are bent and faulted in lines
parallel to the main axis, and also to the chains of volcanic mountains,
and the observation holds good with regard to different geological
periods.[1] Another remarkable case is that of the Jordan Valley.
Nowhere can the existence of a great fracture and vertical displacement
of the strata be more clearly determined than along this remarkable line
of depression; and it is one which is also coincident with a zone of
earthquake and volcanic disturbances.
(_b._) _Such Lines generally lie along the Borders of the Ocean._--But
even where, from some special cause, actual observation on the relations
of the strata are precluded, the general configuration of the ground and
the relations of the boundaries between land and sea to those of
volcanic chains, evidently point in many cases to their mutual
interdependence. The remarkable straightness of the coast of Western
America, and of the parallel chain of the Andes, affords presumptive
evidence that this line is coincident with a fracture or system of
faults, along which the continent has been bodily raised out of the
waters of the ocean. Of this elevation within very recent times we have
abundant evidence in the existence of raised coral-reefs and oceanic
shell-beds at intervals all along the coast; rising in Peru to a level
of no less than 3,000 feet above the ocean, as shown by Alexander
Agassiz.[2] Such elevations probably occurred at a time when the
volcanoes of the Andes were much more active than at present. Considered
as a whole, these great volcanic mountains may be regarded as in a
dormant, or partially moribund, condition; and if the volcanic forces
have to some extent lost their strength, so it would appear have those
of elevation.
(_c._) _Areas of Volcanic Action in the British Isles._--In the case of
the British Islands it may be observed that the later Tertiary volcanic
districts lie along very ancient depressions, which may indicate zones
of weakness in the crust. Thus the Antrim plateau, as originally
constituted, lay in the lap of a range of hills formed of crystalline,
or Lower Silurian, rocks; while the volcanic isles of the Inner Hebrides
were enclosed between the solid range of the Archaean rocks of the Outer
Hebrides on the one side, and the Silurian and Archaean ranges of the
mainland on the other. And if we go back to the Carboniferous period, we
find that the volcanic district of the centre of Scotland was bounded by
ranges of solid strata both to the north and south, where the resistance
to interior pressure from molten matter would have been greater than in
the Carboniferous hollow-ground, where such molten matter has been
abundantly extruded. In all these cases, the outflow of molten matter
was in a direction somewhat parallel to the plications of the strata.
(_d._) _Special Conditions under which the Volcanic Action
operates._--Assuming, then, that the molten matter, forming an interior
magma or shell, is constantly exerting pressure against the inner
surface of the solid crust, and can only escape where the crust is too
weak (owing to faults, plications, or fissures) to resist the pressure,
we have to inquire what are the special conditions under which outbursts
of volcanic matter take place, and what are the general results as
regards the nature of the _ejecta_ dependent on those conditions.
(_e._) _Effect of the Presence or Absence of Water._--The two chief
conditions determining the nature of volcanic products, considered in
the mass, are the presence or absence of water. Such presence or absence
does not of course affect the essential chemical composition of the
_ejecta_, but it materially influences the form in which the matter is
erupted. The agency of water in volcanic eruptions is a very interesting
and important subject in connection with the history of volcanic action,
and has been ably treated by Professor Prestwich.[3] At one time it was
considered that water was essential to volcanic activity; and the fact
that the great majority of volcanic cones are situated in the vicinity
of the oceanic waters, or of inland seas, was pointed to in confirmation
of this theory. But the existence in Western America and other volcanic
countries of fissures of eruption along which molten lava has been
extruded without explosions of steam, shows that water is not an
essential factor in the production of volcanic phenomena; and, as
Professor Prestwich has clearly demonstrated, it is to be regarded as an
element in volcanic explosions, rather than as a prime cause of volcanic
action. The main difficulty he shows to be thermo-dynamical; and
calculating the rate of increase in the elastic force of steam on
descending to greater and greater depths and reaching strata of higher
and higher temperatures, as compared with the force of capillarity, he
comes to the conclusion that water cannot penetrate to depths of more
than seven or eight miles, and therefore cannot reach the seat of the
eruptive forces. Professor Prestwich also points out that if the
extrusion of lava were due to the elastic force of vapour of water there
should be a distinct relation between the discharge of the lava and of
the vapour; whereas the result of an examination of a number of
well-recorded eruptions shows that the two operations are not related,
and are, in fact, perfectly independent. Sometimes there has been a
large discharge of lava, and little or no escape of steam; at other
times there have been paroxysmal explosive eruptions with little
discharge of lava. Even in the case of Vesuvius, which is close to the
sea, there have been instances when the lava has welled out almost with
the tranquillity of a water-spring.
(_f._) _Access of Surface Water to Molten Lava during Eruptions._--The
existence of water during certain stages in eruptions is too frequent a
phenomena to be lost sight of; but its presence may be accounted for in
other ways, besides proximity to the sea or ocean. Certain volcanic
mountains, such as Etna and Vesuvius, are built upon water-bearing
strata, receiving their supplies from the rainfall of the surrounding
country, or perhaps partly from the sea. In addition to this the ashes
and scoriae of the mountain sides are highly porous, and rain or snow can
penetrate and settle downwards around the pipe or throat through which
molten lava wells up from beneath. In such cases it is easy to
understand how, at the commencement of a period of activity, molten lava
ascending through one or more fissures, and meeting with water-charged
strata or scoriae, will convert the water into steam at high pressure,
resulting in explosions more or less violent and prolonged, in
proportion to the quantity of water and the depth to which it has
penetrated. In this manner we may suppose that ashes, scoriae, and blocks
of rock torn from the sides of the crater-throat, and hurled into the
air, are piled around the vent, and accumulate into hills or mountains
of conical form. After the explosion has exhausted itself, the molten
lava quietly wells up and fills the crater, as in the cases of those of
Auvergne and Syria, and other places. We may, therefore, adopt the
general principle that in volcanic eruptions _where water in large
quantities is present, we shall have crater-cones built up of ashes,
scoriae, and pumice; but where absent, the lava will be extravasated in
sheets to greater or less distances without the formation of such cones;
or if cones are fanned, they will be composed of solidified lava only,
easily distinguishable from crater-cones of the first class_.
(_g._) _Nature of the Interior Reservoir from which Lavas are
derived._--We have now to consider the nature of the interior reservoir
from which lavas are derived, and the physical conditions necessary for
their eruption at the surface.
Without going back to the question of the original condition of our
globe, we may safely hold the view that at a very early period of
geological history it consisted of a solidified crust at a high
temperature, enfolding a globe of molten matter at a still higher
temperature. As time went on, and the heat radiated into space from the
surface of the globe, while at the same time slowly ascending from the
interior by conduction, the crust necessarily contracted, and pressing
more and more on the interior molten magma, this latter was forced from
time to time to break through the contracting crust along zones of
weakness or fissures.
(_h._) _The Earth's Crust in a State of both Exterior Thrust and of
Interior Tension._--As has been shown by Hopkins,[4] and more recently
by Mr. Davison,[5] an exterior crust in such a condition must eventually
result in being under a state of horizontal thrust towards the exterior
and of tension towards the interior surface. For the exterior portion,
having cooled down, and consequently contracted to its normal state,
will remain rigid up to a certain point of resistance; but the interior
portion still continuing to contract, owing to the conduction of the
heat towards the exterior, would tend to enter upon a condition of
tension, as becoming too small for the interior molten magma; and such a
state of tension would tend to produce rupture of the interior part. In
this manner fissures would be formed into which the molten matter would
enter; and if the fissures happened to extend to the surface, owing to
weakness of the crust or flexuring of the strata, or other cause, the
molten matter would be extruded either in the form of dykes or volcanic
vents. In this way we may account for the numerous dykes of trap by
which some volcanic districts are intersected, such as those of the
north of Ireland and centre of Scotland.
From the above considerations, it follows that the earth's crust must be
in a condition both of pressure (or lateral thrust) towards the exterior
portion, and of tension towards the interior, the former condition
resulting in faulting and flexuring of the rocks, the latter in the
formation of open fissures, through which lava can ascend under high
pressure. These operations are of course the attempt of the natural
forces to arrive at a condition of equilibrium, which is never attained
because the processes are never completed; in other words, radiation and
convection of heat are constantly proceeding, giving rise to new forces
of thrust and tension.
It now remains for us to consider what may be the condition of the
interior molten magma; and in doing so we must be guided to a large
extent by considerations regarding the nature of the extruded matter at
the surface.
(_i._) _Relative Densities of Lavas._--Now, observation shows that, as
bearing on the subject under consideration, lavas occur mainly under two
classes as regards their density. The most dense (or basic) are those in
which silica is deficient, but iron is abundant; the least dense (or
acid) are those which are rich in silica, but in which iron occurs in
small quantity. This division corresponds with that proposed by Bunsen
and Durocher[6] for volcanic rocks, upon the results of analyses of a
large number of specimens from various districts. Rocks may be thus
arranged in groups:
(1) _The Basic_ (Heavier)--poor in silica, rich in iron; containing
silica 45-58 per cent. Examples: Basalt, Dolerite,
Hornblende rock, Diorite, Diabase, Gabbro,
Melaphyre, and Leucite lava.
(2) _The Acid_ (Lighter)--rich in silica, poor in iron; containing
silica 62-78 per cent. Examples: Trachyte, Rhyolite,
Obsidian, Domite, Felsite, Quartz-porphyry, Granite.
The Andesite group forms a connecting link between the highly acid and
the basic groups, and there are many varieties of the above which it is
not necessary to enumerate. Durocher supposes that the molten magmas of
these various rocks are arranged in concentric shells within the solid
crust in order of their respective densities, those of the lighter
density, namely the acid magmas, being outside those of greater density,
namely the basic; and this is a view which seems not improbable from a
consideration not only of the principle itself, but of the succession of
the varieties of lava in many districts. Thus we find that acid lavas
have been generally extruded first, and basic afterwards--as in the
cases of Western America, of Antrim, the Rhine and Central France. And
if the interior of our globe had been in a condition of equilibrium from
the time of the consolidation of the crust to the present, reason would
induce us to conclude that the lavas would ultimately have arranged
themselves in accordance with the conditions of density beneath that
crust. But the state of equilibrium has been constantly disturbed. Every
fresh outburst of volcanic force, and every fresh extrusion of lava,
tends to disturb it, and to alter the relations of the interior viscous
or molten magmas. Owing to this it happens, as we may suppose, that the
order of eruption according to density is sometimes broken, and we find
such rocks as granophyre (a variety of andesite) breaking through the
plateau-basalts of Mull and Skye, as explained in a former chapter.
Notwithstanding such variations, however, the view of Durocher may be
considered as the most reasonable we can arrive at on a subject which is
confessedly highly conjectural.
(_j._) _Conclusion as regards the Ultimate Cause of Volcanic
Action_.--Notwithstanding, however, the complexity of the subject, and
the uncertainties which must attend an inquiry where some of the data
are outside the range of our observation, sufficient evidence can be
adduced to enable us to arrive at a tolerably clear view of the ultimate
cause of volcanic action. So tempting a subject was sure to evoke
numerous essays, some of great ingenuity, such as that of Mr. Mallet;
others of great complexity, such as that of Dr. Daubeny. But more recent
consideration and wider observation have tended to lead us to the
conclusion that the ultimate cause is the most simple, the most
powerful, and the most general which can be suggested; namely, _the
contraction of the crust due to secular cooling of the more deeply
seated parts by conduction and radiation of heat into space_. Owing to
this cause, the enclosed molten matter is more or less abundantly
extruded from time to time along the lines and vents of eruption, so as
to accommodate itself to the ever-contracting crust. Nor can we doubt
that this process has been going on from the very earliest period of the
earth's history, and formerly at a greater rate than at present. When
the crust was more highly heated, the radiation and conduction must have
been proportionately more rapid. Owing to this cause also the
contraction of the crust was accelerated. To such irresistible force we
owe the wonderful flexuring, folding, and horizontal overthrusting which
the rocks have undergone in some portions of the globe--such as in the
Alps, the Highlands of Scotland and of Ireland, and the Alleghannies of
America. It is easy to show that the acceleration of the earth's
rotation must be a consequence of such contraction; but, after all, this
is but one of those compensatory forces of which we see several examples
in the world around us. It can also be confidently inferred that at an
early period of the earth's history, when the moon was nearer to our
planet than at present, the tides were far more powerful, and their
effect in retarding the earth's rotation was consequently greater.
During this period the acceleration due to contraction was also greater;
and the two forces probably very nearly balanced each other. Both these
forces (those of acceleration and retardation) have been growing weaker
down to the present day, though there appears to have been a slight
advantage on the side of the retarding force.[7]
[1] R. D. M. Verbeek, _Krakatau_, p. 105 (1886); also, J. Milne, _The
Great Earthquake of Japan_, 1891.
[2] _Bull. Mus. Comp. Zool._, vol. iii.
[3] _Proc. Roy. Soc._, No. 237 (1885); also, _Rep. Brit. Assoc._ (1881).
[4] Hopkins, _supra cit._, p. 218.
[5] C. Davison and G. H. Darwin, _Phil. Trans._, vol. 178, p; 241.
[6] Durocher, _Ann. des Mines_, vol. ii. (1857).
[7] See on this subject the author's _Textbook of Physiography_ (Deacon
and Co., 1888), pp. 56 and 122.
CHAPTER II.
LUNAR VOLCANOES.